Method of annealing type 430 stainless steel



Dec. 4, 1962 w. B. LEFFINGWELL ETAL 3,067,072

METHOD OF ANNEALING TYPE 430 STAINLESS STEEL Filed NOV. 7, 1960 mic. m/M/ f zmzw zuzw /FELMSE/ ill.. y|i\ 1111 Il@ DE oztzm :22.2.: w lllllml.. 2%?

lllll IIV 60o- 8E OQ! 8m. O8. 80N CONN INV EN TORS Wallace B.Le

al vans it Amm United States Patent 3,057,072 METHOD F ANNEALTNG TYPE430 STAINLESS STEEL Wallace B. Lengwell and Henry G, Evans, Sharpsville,Pa., assignors to Sharon Steel Corporation, Sharon, ifa.,

a corporation of Pennsylvania Filed Nov. 7, 1960, Ser. No. 67,329 2Claims. (Cl. 14S- 12) The invention relates to the preparation ortreatment of AISI Type 430 stainless steel strip. More particularly, itpertains to a method of annealing hot rolled Type 430 stainless steelstrip preliminary to subsequent cold rolling.

Type 430 stainless steel is a ferritic alloy of a group of alloys havingthe basic chemical constituents of 0.12% maximum carbon and of from 14.0to 18.0% chrorniurm Such alloys, which are also referred to as ferriticstainless steels or chromium steels, are non-hardening and ferritic attemperatures up to about l650 F.; and, being somewhat easily formed, areused for automobile trim, chemical equipment, and the like. Indeed, thealloys can be hot or cold Worked without undue detriment to theirphysical properties, grain structure, and corrosion resistance.

An outstanding characteristic of Type 430 stainless steel is that it isferritic at temperatures up to l1650c F. Above -that temperature itpartiallyy transforms into austenite which upon air cooling to roomtemperature transforms into martensite. Prior to cold rolling, thestainless steel strip must be annealed because of the relatively highhardness of about Rockwell B l00'that results from air cooling the hotrolled material.

The conventional method for annealing Type 430 stainless steel has beensomewhat complicated, time-consuming and costly. In this conventionalmethod, the hot rolled strip, after being air cooled to roomtemperature, is reheated into a temperature range of 1550 F. to 1600 F.where it is held for approximately thirty minutes. The material is thenfurnace cooled to 145 0 F. where it is held for `four hours untilspheroidization is complete. Higher temperatures of annealing areavoided ecause of the hazard of pearlite formation rather thanspheroidized crabides. Thereafter the material is cooled to y1100" F.and then air cooled to room temperature. That annealing procedure is notonly time-consuming but it wastes heat and it is therefore costly.

We have found a more efficient and less costly annealing process, inwhich the heat present in the steel after hot rolling at a finishingtemperature of about l900 F. and before other cooling, is utilized toanneal the strip by a controlled cooling cycle `from the hot rollingfinishing temperature to a temperature within the transformation range,the strip temperature being maintained lfor a period of time below`austenizing temperature and above 1400 F. to perform isothermalannealing. More particularly, the material is transferred immediatelyafter hot rolling to a furnace for controlled isothermal annealing at alower temperature of about 1450 F. (below the austenizing temperature)and held for a suiiicient time to insure complete transformation ofaustenite to ferrite and spheroidized carbides and then air cooled toroom temperature. isothermal annealing has 'been defined as a process inwhich a ferrous alloy "ice is heated to produce a structure that ispartly or wholly austenitic, and then cooled to and held at atemperature that causes transformation of the austenite to a relativelysoft ferrite-carbide aggregate. Such a procedure, if properly integratedwith hot rolling facilities, reduces processing time by several days andsaves most of the fuel that is otherwise normally required to reheat thecold strip as in conventional annealing.

Type 430 stainless steel responds to heat treatment in a mannerdifferent from other types of steel. Some steels are not susceptible toisothermal annealing. Low carbon steels are usually air cooled as fastas possible after hot rolling. High carbon steels will form the familiarlamellar pearlite structure upon cooling from hot rolling temperatures.Similarly, Type 406 steel, a ferritic stainless steel, having about 13%chromium, cannot be compared with Type 430 stainless steel because Type406 has a completely ferritic matrix during hot rolling and forms a moresoft ferrite-carbide aggregate upon annealing than does Type 430stainless steel.

Type 446 steel, which is a ferritic stainless steel, also cannot becompared with Type 430 steel because of the higher (23.0 to 27.0%)chromium content of Type 446 which renders this steel completelyferritic at hot rolling temperature. Moreover, since Type 446 containsno austenitic phase at hot rolling temperatures as does Type 430,annealing of the hot strip presents an entirely different problem.Accordingly, because of the varying characteristics of stainless steelssuch as Types 406, 430 `and 446, a prediction as to the behavior of oneof them, based upon behavior of another, cannot be made.

Nor can the conventional practice of box annealing hot rolled strip becompared with isothermal annealing procedure immediately following hotrolling. Furthermore, all known methods of slow cooling after hotrolling are not sufficient to produce the particular results obtained byisothermally annealing Type 430 steel directly after hot rolling. Suchslow cooling methods simply do not hold the heat in the strip in therequired temperature range long enough to permit the isothermaltransformation to occur. Such a transformation is the process oftransforming austenite in a ferrous alloy to a ferrite-carbide aggregateat a constant temperature within the transformation range.

Accordingly, it is a general object of this invention to provide anisothermal annealing method in which Type 430 stainless steel isisothermally annealed immediately after hot rolling at a temperaturewithin the transformation range for a sufficient length of time toproduce a spheroidal carbide structure.

lt is another object of this invention to provide an isothermalannealing treatment for Type 430 steel which results in physicalproperties more conducive to subsequent cold working.

lt is another object of this invention to provide hot rolled Type 430stainless steel strip products having lower strength and higherductility for subsequent cold Working, as well as more uniform tensileproperties measured in the transverse and longitudinal directions thanheretofore present in Type 430 stainless steel strip products.

It is another object of this invention to provide an isothermalannealing procedure for Type 4-30 stainless steel which saves time andfuel, thereby substantially reducing production costs.

acer/,07a

Finally, it is an object of this invention to provide a method ofannealing Type 430 stainless steel which eliminates prior artdifliculties in conventional annealing methods, which improves theultimate cold working procedures and products and which obtains theforegoing advantages and desiderata in a simple and effective inanner.

These and other objects may be accomplished by the methods, steps,procedures, principles, and treatments comprising the present invention,the nature of which is set forth in the following general statement, anda preferred embodiment of which-illustrative of the best mode in whichapplicants have contemplated applying the principlesis set forth in thefollowing description and illustrated in the accompanying drawing, andwhich is particularly and distinctly pointed out and set forth in theappended claims forming a part hereof.

The improvements, in a method of providing cold working Type 430stainless steel `strip products, of the present invention, may be statedin general terms as comprising holding the temperature of the hot rolledsteel, immediately after hot rolling to strip form of the desiredthickness, and while still retaining heat from the heating for hotrolling, in a temperature range of from 1400 F. to 1500 F. in a furnacefor a minimum of three hours, and then air cooling to room temperatureto provide a ferrite stainless steel having a spheroidized carbidestructure in a ferrite matrix.

Generally the chemical analysis of Type 430 stainless steel materialtreated in accordance with the invention may be as follows:

Carbon .12% max.

Manganese 1.00% max. Silicon 1.00% max.

Nickel .50% max. Chromium 14% to 18%.

In addition, traces of other elements such as P, S and possibly .smallamounts of Ti, N, Mo, etc. may exist as incidental impurities. A typicalType 430 analysis is as follows:

Percent Carbon .055

Manganese .35 Silicon .27

Nickel .19

Chromium 16.54

Conventional yoperations heretofore used for producing Type 430 coldrolled steel strip include the following steps:

(1) Hot roll iat from 2300" F. to 1900o F. (2) Air cool to roomtemperature. (3) Reheat to 1550 F. to 1600 F. for 30 minutes. (4)Furnace cool to 1450D F. and hold for four hours. (5) Furnace cool to1100 F. and then air cool to room temperature. (6) Pickle in hotnitric-hydroiluoric acid. (7) Cold roll by 10% reduction passes from.150 to .060

inch thick strip.

(8) Stress relief anneal at 1400 F. (9) Pickle.

This conventional procedure is altered in accordance with the inventionas follows:

(1) Hot roll at from 2300" F. to 1900 'F. (2) While still retaining hotrolling heat, furnace cool to and isothermally furnace anneal at atemperature between 1400 F.1500 F. for at least three hours. Air cool toroom temperature. Pickle in hot nitric-hydroiluoric acid. Cold roll by10% reduction passes from .150 to .060 inch thick strip.

(6) Stress relief anneal at 1400D F. (7) Pickle.

In accordance with the invention, Type 430 `stainless steel aftermelting and casting in the usual manner is processed by hot rolling toforni the desired semi-finished het rolled strip product. For instance,ingots of the alloy are hot rolled to coils of semi-finished striphaving thicknesses of .090 to .120" in a temperature range of 1900 F. to2300 F. The coils of strip are then transferred immediately while stillretaining hot rolling heat to an annealing furnace and treated asoutlined in Treatment A below.

Hot rolled Type `430 strip steel was subjected to three differentannealing treatments, shown diagrammatically in the drawing, in order toevaluate and compare the properties of material treated in accordancewith the invention and material treated by conventional procedures.These treatments yare referred to herein as treatment A, treatment B andtreatment C as follows:

(A) isothermal annealing treatment in accordance with the invention,wherein the strip was transferred, immediately after hot rolling andwhile still retaining hot rolling heat, to a gas-fired tunnel furnacewhere it was isothermally annealed for three hours at 1450 F.i25 F.,then air cooled to room temperature.

(B) Conventional annealing treatment wherein the strip was air cooledafter hot rolling, then reheated to 16C-0 F. for 30 minutes, thenfurnace cooled to 1450 F. for four hours, then furnace `cooled to 1100F., and then air cooled to room temperature.

(C) Special isothermal annealing treatment wherein the strip was aircooled after hot rolling, then reheated to 1900 F. for 30 minutes, thentransferred to an annealing furnace at 1450 F. for three hours, landthen air cooled to room temperature.

After being subjected to each of the three treatments Af B and (2, thestrip material was pickled in a solution containing 20% nitric and 5%hydrofluoric acid. Thereafter the `strip was cold rolled to .060-.065inch thick strip, after which it was annealed for stress relief at 1400F. for one hour, and pickled. The strip was then cold rolled to about.020 inch thick, annealed for stress relief at 1440" iF., and pickled.Finally, the strip was cold rolled from .020 to .O10 inch, and stressrelief annealed for 15 minutes at l450 F.

The results of hardness tests on the isothermally (treatment A) andconventionally (treatment B) annealed Type 430 strip in variousconditions yare shown in rliable l:

TABLE I Haiidness of Isothernially and Conventon'irlly Aiziiealed Type430 Strip in Various Conditions Rockwell B hardness Condition of stripIsother- Convenmally tionally annealed annealed Hot rolled to 0.120inch, air cooled 96. 5 Hot rolled aud annealed 79. 5 81.0 Cold rolled to0060-0070 inch..- 97. 5 98.0 Stress relieved at 1,400o F... 72. 5 76.0Cold rolled to 0.035 inch 99.0 99. 0

Cold rolled t0 0.020 inch l 101 l 100 Stress annealed at 1,400" F l 82 l82 Cold rolled t0 0.0l0 inch 1 102 l 103 Stress relieved at 1,450 F. 185 l 88 lHardness measured on Rockwell 15T scale and converted toRockwell B values.

In general, the isothermally annealed strip in both the as-rolled andas-annealed conditions is softer than the conventionally annealed strip.In the as-rolled condition the hardness increases gradually as the stripthickness is decreased by cold rolling. In the annealed orstress-relieved condition, lower hardness values are obtained for.060-.070 inch cold rolled strip than for the .10S-.115 inch hot rolledstrip. Subsequent reduction by cold rolling from .G60-.070 inch to .010inch thickness, however,

results in successively higher hardness Values for stressrelievedmaterial. This gradual increase in hardness was probably caused by adecrease in grain size.

The results of tensile tests on the isothermally (treatment A) andconventionally (treatment B) annealed Type 430 strip in variousconditions are given in Tables II and III below, the data being theaverage for four test specimens in each instance:

TABLE II Tensile Properties of lsohermally Annealed Type 430 StainlessStrip (Treatment A) HOT-ROLLED (0100-0110 IN.) ISOTHERMALLY ANNEALEDTensile Properties of Conventionally Annealed Type 430 Stainless Strip(Treatment "B) HOT-ROLLED (0090-0100 IN.) CONVENTIONALLY AN' NEALEDYield Tensile Elongation, Direction of testing strength strength,percent in with respect to rolling 0.2 percent ps1. 2 inches otset,p.s.i.

Longitudinal 45, 600 68, 150 24 Transverse 50, 100 75, 400 19COLD-ROLLED (0.065 IN.), STRESS-RELIEVED (1,400 F.)

Longitudinal Transverse OOLD-ROLLED (0.020 N.), STRESS-RELIEVED (ifi-t0"F.)

Longitudinal 43, 100 67, 600 31 COLD-ROLLED (0.010 IN1), STRESS-RELIEVED(1,450" F.)

Lon gitudinal 41, 200 65, 700 28 In general, the isothermally annealedstrip exhibits (Table Il) lower yield strength and tensile strength andhigher ductility than does the conventionally annealed strip (Table III)for any given strip thickness. Moreover, in both cases the strengths arehigher and the ductility is lower when measured in the directiontransverse to cold rolling.

The data in Tables Il and III further reveals that the most pronounceddiierence in the tensile properties of isothermally and conventionallyannealed strip occurs in the hot rolled and annealed specimens, and thatsubsequent cold rolling and stress relieving gradually reduces thedifferences to an almost insigniiicant degree. For example, in thehot-rolled and annealed condition, the isothermally annealed strip showsaverage tensile strength Values of 63,600 p.s.i. in the longitudinaldirection and 66,150 p.s.i. in the transverse direction withcorresponding values for ductility of 26 and 24%. The conventionallyannealed strip shows appreciably higher tensile strength values of68,15'0 p.s.i. in the longitudinal direction and 75,400 p.s.i. in thetransverse direction with respective ductility values of 24 and 19%.

After cold rolling to about 0.064 inch thick strip and stress relievingat 1400 F., the isothermally annealed specimens show nearly the sametensile strength values in both directions as before, but ductility hadincreased to 34% in the longitudinal and to 32% in the transversedirection. Conventionally annealed strip shows about the same tensilestrength in the longitudinal direction as before (68,000 p.s.i.), asignificantly lower tensile strength in the transverse direction (71,000p.s.i.), and higher ductility values of 30% in the longitudinal and 28%in the transverse direction. After further cold rolling to 0.020 inchthick strip and strand annealing at 1400 F., only slight dilferences arefound in tensile strength (66,300 p.s.i. and 67,000 p.s.i.) andductility (33% and 31%) values of isothermally and conventionallyannealed strip.

In order to determine whether there would be a distinguishabledifference in tensile properties of Type 430 strip which, instead ofbeing annealed isothermally at 1450lu F. immediately after hot rolling,was allowed to cool to room temperature, then reheated to within theaustenitic range of 1900 F., and nally annealed isothermally at 1450L7F., a group of samples were tested after treatment in this manner, thetreatment being referred to heretofore as treatment C. The tensile data(each value being the average of four tests) on 0.060 inch thick coldrolled and stress relieved strip are shown in Table IV.

TABLE IV Tensile Properties of Hot-Rolled, Air-CooledIsothermally-Annealed Type 43,0 Stainless Strip 1 COLD-ROLLED(0.060-INCH) STRESS-RELIEVED (1,400" F.)

l Reheated to l900 F. for 30 minutes, isothermally annealed for 3 hoursat 1,450 F.

When the data in Table IV is compared with the results of otherwisesimilarly processed strip in Table II, no significant difference intensile properties is present. Thus, similar tensile properties wouldapparently result if Type 430 strip were isothermally annealed eitherimmediately after hot rolling (treatment A) or after cooling to roomtemperature and then reheating (treatment C) to l900 F., the iinishingtemperature for hot rolling. However, treatment C involves a reheatingoperation and is therefore more costly and time-consuming.

Metallographic studies made to compare the structures of Type 430stainless steel in the hot rolled and isothermally and conventionallyannealed conditions, as well as in the stress-relieved condition aftercold rolling to .065, .020, and .010 inch thick strip reveal structuralsimilarities and diilerences which appear to be related either to theannealing treatment or to the rolling practice, or both.

Structural similarities consist of ferrite grains (which were the resultof austenite transformation), a matrix of delta ferrite, and carbideparticles which had precipitated at prior austenite grain boundaries andwithin the delta ferrite matrix. Structural differences exist in thesizes of the ferrite grains and the amount and distribution of theprecipitated carbide particles.

In the hot rolled and annealed condition, the prior austenite grains inisothermally annealed strip are noticeably larger than those in theconventionally annealed strip. yIn both cases the ferrite grains appearas thin, elongated lgrains surrounded by carbides, and randomly orientedwith respect to the rolling direction. No distinguishable differenceexists in the nature of the precipitated carbides that could beattributed to annealing practice.

A visualcomparison'of isothermally and conventionally annealed Type 430stainless strip after cold rolling and subsequent strip relieving andpickling shows no distinguishable differences in surface appearancewhich can be attributed to either of the two annealing practices.

Briefly, in -accordance with the invention and on the basis of hardnessdata, tensile properties, microstructure and behavior during subsequentcold rolling, Type 430 stainless strip should be isothermally annealedimmediately after hot rolling within the temperature range of 1400 F. to1500 F. for `at least three hours. Under those conditions the hardnessis low (about Rockwell B75), the tensile and yield strengths are low,ductility is. high, the structure consists of ferrite (delta ferriteplus ferrite produced from decomposition of austenite) land precipitatedcarbides, and cold rolling behavior is very satisfactory as evidenced bya relatively low degree ofwork hardening.

When strip is isothermally annealed at a lower ternperature of say 1300oF., pearlite is found in the prior aus-tenite grains. Annealing for lessthan three hours at 1400 IF. or 1450 F. results in an incompletedecomposition of austenite. In both instances the hardness afterannealing is higher and the degree of work hardening and susceptibilityto cracking during cold rolling are greater than for the preferredconditions.

In accordance with the invention, an isothermal annealing treatment forAISI Type 430 stainless steel strip within a temperature range of 1400F. to 1450 F. irnmediately after hot rolling can be effectively employedin processing Type 430 strip. The optimum treatment for isothermalannealing is in the temperature range of 1400 F. to 1450 F. for threehours or more. Isothermally annealing strip immediately after hotrolling results in lower hardness than is produced by conventionalanneal- Ving. At the same time, lower strengths and higher ductilityresult from isothermal annealing as `compared with conventionalannealing. Although the isothermally annealed strip has lower hardnessand strength and higher ductility than conventionally annealed strip,the differences in properties diminishes as the strip is further coldrolled from .065 to .01 inch thick strip and stress relieved.

Moreover, isothermally annealed strip may be cold rolled as easily Iasconventionally annealed strip, resulting in -a surface quality equal tothat of conventionally annealed strip at thicknesses ranging from .125to .010 inch.

Finally, the isothermal annealing procedure of the invention for Type430 steel strip is commercially desirable because it results in asavings in operating costs and time with no sacrifice in rollingbehavior, tensile properties, or surface characteristics.

In the foregoing description, certain terms have been used for brevity,clearness and understanding; but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchterms are utilized for descriptive purposes Iherein and not for thepurpose of limitation and are intended to be broadly construed.

Moreover, the description of the improvements is by w-ay of example andthe scope of the present invention is not limited to the exact detailsillustrated.

Having now described the features, discoveries and principles of theinvention, the procedures or steps necessary to accomplish theobjectives, the characteristics of the new products obtained thereby,and the advantageous, new and useful results provided; the new anduseful methods, steps, operations, procedures, discoveries andprinciples, and mechanical equivalents obvious to those skilled in theart, are set forth in the appended claims.

What is claimed is:

1. The method of making hot rolled annealed Type 430 stainless steel athot rolling temperature range of 2300 F. to 1900 F. to form hot rolled,soft, ductile, fully spheroidized strip that is completely free ofmartensite; furnace cooling the hot rolled strip from the hot rollingtemperature range immediately after hot rolling and while stillretaining not rolling heat to a temperature in the range of 1400 F, to1500" F., maintaining `the strip at said 1400 F. to 1500 F. temperaturefor at least three hours; and then cooling the hot rolled strip to roomtemperature.

2. The method set forth in claim l in which the hot rolled strip isfurnace cooled to and maintained at 1450o F. for at least three hours.

References Cited in the le of this patent UNITED STATES PATENTS2,851,384 Waxweiler Sept. 9, 1958 OTHER REFERENCES Republic EnduroStainless Steels, pp. 37-39, 1951. The Book of Stainless Steels, 2nded., pp. 328429, 1935.

1. THE METHOD OF MAKING HOT ROLLED ANNEALED TYPE 430 STAINLESS STEEL ATHOT ROLLING TEMPERATURE RANGE OF 2300* F. TO 1900*F. TO FORM HOT ROLLED,SOFT, DUCTILE, FULLY SPHEROIDIZED STRIP THAT IS COMPLETELY FREE OFMARTENSITE; FURNACE COOLING THE HOT ROLLED STRIP FROM THE HOT ROLLINGTEMPERATURE RANGE IMMEDIATELY AFTER HOT ROLLING AND WHILE STILLRETAINING HOT ROLLING HEAT TO A TEMPERATURE IN THE RANGE OF 1400*F. TO1500*F., MAINTAINING THE STRIP AT SAID 1400*F. TO 1500*F. TEMPERATUREFOR AT LEAST THREE HOURS; AND THEN COOLING THE HOT ROLLED STRIP TO ROOMTEMPERATURE.